The injection molding of relatively deep parts, such as waste baskets or other open-top containers, utilizes a mold having a cavity defining the exterior of the part, and a core, which extends into the cavity and defines the interior of the part. Heated thermoplastic is injected between the cavity and the core to form the part. Typically, the molded part is retained on the core as the core is removed from the cavity. Thereafter, ejectors separate the part from the core. This is also known as "stripping" the part.
The mold is, of course, cooler than the plastic and receives heat transferred from the plastic so that the plastic will set in the desired shape of the molded part. Molds usually have coolant passages for circulating coolant in order to remove heat from the mold and solidify the plastic part. As the molded part cools, it contracts and attains a tight, gripping fit on the mold core. In the case of a relatively deep molded part, such as a waste basket or other part having a sidewall of substantial height, the contraction of the part may cause difficulty in removing the part from the core. More particularly, the part shrinks to the core and therefore requires substantial force to release it. The molding machines generally contain part ejectors for applying force to the open end or lip of the part, with the object of axially removing the part from the core. However, if the part has shrunk too tightly to the core, the sidewall of the part may deform in the course of attempting to remove the part, leaving stress marks on the side of the part. When the part wrinkles rather than releasing cleanly from the core, the part must be rejected and scrapped.
Although some relief from this problem may be obtained by proper lubrication of the core, for example, with zinc stearate, it is usually necessary to provide the part with a substantial draft angle. The draft angle is the angle at which the part widens from its base to its lip. There are various formulas available in the molding art for calculating the draft angle for a particular part. A typical draft angle for a 20 inch high waste basket is 11/2.degree., and is often greater if a textured surface is provided.
Although some draft angle is often desirable, e.g., so that the completed parts may nest or partially nest for shipping and storage, it is often necessary to increase the draft angle beyond that necessary for nesting. Thus, some parts are not as useful as they might be. For example, containers having a smaller base area than top opening area may be top heavy, or may have less than the desired volume given the footprint on a surface, the footprint being determined by the top size. Other parts, for example, a deep box with substantially vertical sidewalls, simply cannot be made because of the requirement of a draft angle in the molding process.
An alternative to high draft angles is to withdraw the core while leaving the part in the cavity. The bond between the part and the core can be broken by use of compressed air or the like, which will not distort the part while the part is retained in and supported by the cavity. However, this process requires an extra step of removing the part from the cavity.
As discussed above, some parts have a tendency to be retained on the mold core, but some parts have a tendency to be retained in the cavity rather than on the core. This is particularly true of parts having a textured exterior surface which, as the part begins to harden, forms an interference fit with the surface of the cavity. Such textured parts may not be released and withdrawn with the core, and may also be difficult to remove from the cavity by cavity ejectors. Again, the art teaches increased draft angles as the solution, adding 1.degree. to 11/2.degree., of draft angle for each 0.001" of texture or other pattern depth.
Other parts may, by nature of their configuration, also tend to be retained within the mold cavity or retained on the mold core in such a manner that ejection of the part causes damage. In some instances, a longer cooling period before ejection protects the part, but costs cycle time. Therefore, there is a distinct need for a molding process which is capable of producing a part with a low draft angle, textured surfaces and the like, and which reliably removes the part from the mold without damage to the part, at efficient cycle times.